When cleaning municipal and commercial sewage by way of mechanical-biological processes, sewage sludge is produced which must be disposed of or returned to the material cycle, respectively. Sludge digestion, as a standard method of biological sludge stabilization, can there lead to a significant reduction in the dry sludge matter. Apart from obtaining recyclable sewage gas, the reduction of dry sludge matter to be disposed of gives rise to economic advantages for the operation of sewage treatment plants. However, conversion of the organic substances from activated sludge is generally limited to 25 to 30%, since they consist largely of a poorly degradable organic cell substance.
A method for the production of biogas is known from WO 2007/117152 A1, wherein heating of organic matter in a digester by way of circulation through a heat exchanger is disclosed therein. Fertilizer is gained in a dewatering device, where parts of the dehydrated residue or the liquid residue can be returned.
Disintegration systems for the destruction of the cell substance contained in sewage sludge are in part provided in prior art, by way of which the degree of degradation and the gas yield can be significantly increased and the dry sludge mass to be disposed of can be reduced.
Sewage sludge disintegration is in prior art defined as the comminution of sewage sludge by the action of external forces (physical, chemical, biological). It leads from the dissolution of the flake structure to the digestion of the microorganisms and particles contained in the sludge. Sewage sludge disintegration therefore also comprises the hydrolysis of sewage sludge, where these terms are often used synonymously in prior art and hereinafter.
A device and a method according to the preamble are known, for example, from EP 2 233 442 A1 It is therein further disclosed that a recirculation line and a recirculation valve are provided through which at least part of the organic matter can be recirculated directly after hydrolysis in order to compensate possibly discontinuously occurring initial quantities of the organic matter, thereby being able to maintain continuous operation of the system. In addition, this recirculation line enables cleaning liquid to be circulated through the system in order to therewith remove possible incrustations from the organic matter in the system.
EP 0 996 595 B1 discloses a method for the treatment of biogenic residual masses in which a reactor heated by hot thermal oil for temperature-pressure hydrolysis is provided, where the slurry leaving the reactor is fed to a fermenter via a stripping column and a device for withdrawing hydrolysis gas. The resulting turbid water can for the production of new slurry be fed to a mashing tank, where the solids content in the turbid water is further reduced by way of a static thickener.
EP 1 150 924 B1 and DE 198 58 187 C 5 describe methods and devices for reducing the organic content of a digestible substrate. In one embodiment, excess sludge is first fed to a digestion tower. Sludge is then removed from the digestion tower by way of a line and is via a thickener and a heat exchanger for thermal disintegration again returned to the digestion tower.
DE 10 2007 037 202 A1 discloses a method for the conversion of biomass into biogas in anaerobic fermenters, where the fermentation residue is downstream of a first fermenter subjected to a solid-liquid phase separation and the separated solid phase is subjected to thermal pressure hydrolysis before it is either returned into the first fermenter reactor or fed to a second fermenter reactor. A steam-operated thermal hydrolysis device is provided upstream of the digester.
It is in WO 92/13084 A1, WO 88/04282 A1 and EP 0 142 873 A1 respectively disclosed that only the liquid portion of the hydrolyzed organic matter is fed to a digester.
If in sewage sludge treatment devices with an upstream hydrolysis system or a disintegration system, respectively, a sludge treatment component, for example a thickener, fails or needs to be serviced upstream of the hydrolysis system, then any organic matter, e.g. thickened sludge, is no longer fed during this time. Consequently, the hydrolysis system would have to be shut down. However, this is disadvantageous because re-starting the hydrolysis system from the cold state can take a very long time, for example up to 16 hours. For this reason, a storage for the organic matter can be provided upstream of the hydrolysis system and buffer sufficient organic matter, such that sufficient organic matter is even in the event of a longer downtime of an upstream component available for the operation of the hydrolysis system and the hydrolysis system must only thereafter in the extreme case be shut down in a controlled manner.
However, such a storage container is expensive and requires much space. With an assumed storage time of one day for a medium-sized sewage treatment plant with a cleaning capacity for a 200,000 population equivalent, such a storage tank would have the size of about 120 m3 (cubic meters).